SI Unit of Surface Tension

Liquid substances stay compact when they accumulate in small droplets. This phenomenon depends on the inward attractive forces among the liquid particles that act simultaneously in addition to the adhesive forces between the water molecules and their surrounding object. The surface tension is active at the topmost layer of a liquid where it interacts with the air. It is the intrinsic force that acts tangentially stretching the liquid outwards. This exhibits elasticity. Force per unit length gives the dimension of surface tension.

There are strong electrostatic attractive forces within the network of hydrogen atoms that structure together to form water droplets. This causes minimization of the occupied surface area. This causes a higher surface tension of water when compared to the rest of the liquids.

Basis of Surface Tension

Within a liquid body, all the molecules are subjected to cohesive forces of attraction. The omnidirectional forces nullify the net force. The inherent cohesion does not apply to the molecules that are present on the surface. Excessive internal pressure is generated as we travel away from the surface. This is because the topmost molecules do not interact with similar particles on all sides. Hence, liquids tend to contract. The tangential surface tension per unit area resists this cohesiveness. Both the forces ensure to contribute to the elasticity of water drops. Surface tension unit is given as Newton per meter.

As the interior molecules of a liquid are surrounded by neighboring liquid molecules in all directions, they conserve less amount of energy than their boundary counterparts. Thus smaller numbers of particles that are subjected to adhesion help in the minimization of the surface area. The boundary elements get attracted inwards by a reaction force to assume the smoothest definition. The accumulative forces that act on the plane of a liquid surface per unit length area define surface tension.

In Physics, Laplace’s Law is there to define surface tension. The existing pressure across the surfaces of two immiscible fluids is known as capillary pressure. The wetting properties of a liquid on a surface depend on the capillary pressure (pc). After a liquid is poured into a vessel, the meniscus is the air-liquid interaction level. It appears to be concave when the degree of wetting is greater than the cohesion energy and other forces among the water molecules like Van Der Waals force. On the contrary, if the adhesive forces between the liquid and its vessel or any other adjoining medium tend to be less than half of its cohesion, the topmost layer appears as convex. Each liquid surface presents a spherical outlook resulting from a cohesive imbalance in the surface.

Define Surface Tension occurrences in real world

1. Tears of wine are visible on the liquid surface of an alcoholic drink. Drops constantly fall inwards. This complex phenomenon practically helps to define surface tension. It results due to the varying surface tensions of the more volatile ethanol and water.
2. When water runs down from a faucet, the gravitational force causes the water to stretch. This flattens the surface. The water stream disintegrates into drops when the surface tension fails to link the mass of water to the faucet. The drops of water exhibit minimization of the meniscus.
3. Insects like water striders walk on the water surface as they are denser in the buildup. Surface tension supports their feather-light weight, so they do not sink. Here, the water molecules simply try to retain their original spherical shape that was deformed by the legs of the water strider. This behavior generates sufficient tension to keep the insect floating.

Viscosity

Viscosity is defined as a measure of molecular rigidness in the flow of liquids. Fluidity is the opposite property of viscosity. Internal friction among the liquid molecules slows down the velocity of its flow.

The rate of deformation of any liquid is directly proportional to the surface tension of the liquid at a particular temperature. Here, the constant involved is known as dynamic viscosity. It is different from kinematic viscosity which is measured with the help of the absolute density of a fluid.

Kinematic density = absolute viscosity / fluid’s mass density

Newtonian fluids which have lower viscosity can flow easily because their molecular build up offers a very less amount of resistance. This is the reason why honey cannot flow in the same manner as water.

Viscosity Formula

The viscosity of a fluid is measurable concerning a sphere dropped in that fluid. The viscosity formula is the ratio of applied perpendicular force on a fluid surface (shear stress) to that fluid’s velocity gradient.

V = r²2g (∆ ρ) /9v

Here, ∆ ρ gives us the difference between the densities of the sphere and the fluid. The radius of the object is ‘r’. The velocity of the dropped sphere is represented by ‘v’ while ‘g’ is the acceleration due to gravity.

The S. I. Unit of viscosity is newton-second per m2.

Conclusion

To define surface tension, we must address the attribute of any liquid that helps it to assume minimum free surface. Due to this physical property, the liquid behaves as an elastic membrane. Viscosity offers resistance to the flow of liquids over a solid surface.